feat: Safe Git Operations Interface (GitOps)

[RevCBH]

Summary

• Introduces a new GitOps interface that provides safe, validated git operations bound to specific repository paths
• Adds MockGitOps for testing with assertion helpers and safety simulation
• Migrates the worker package to use GitOps instead of raw git.Runner calls

Problem

The previous pattern of passing directory paths directly to git.Runner.Exec() was error-prone:

• Empty dir string would silently run commands in the current working directory
• A production bug caused tests to inadvertently run destructive git commands (git checkout ., git reset, git clean -fd) on the actual repository instead of test directories
• No type safety to prevent passing the wrong path

Solution

The new GitOps interface:

• Path binding: Each instance is bound to a specific repository path at construction time
• Validation: Path is validated to be non-empty and to exist before any operation
• Per-repo locking: Write operations acquire locks to prevent concurrent modifications
• Rich result types: Structured return values (e.g., StatusResult, MergeResult) instead of raw strings
• Mock support: MockGitOps enables precise test control with safety simulation and assertion helpers

Test plan

• Unit tests for GitOps implementation covering all operations
• Unit tests for MockGitOps assertion helpers
• Integration tests for git operations in real repositories
• Worker tests migrated to use MockGitOps
• CI passes

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